This section is intended to provide a background or context to the invention that is, inter alia, recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
A nanofluid generally refers to a liquid mixture with a small concentration of nanometer-sized (about 1 to 500 nm length scale) solid particles in suspension. Nanoparticles are typically made of chemically stable metals, metal oxides or carbon, in various forms. Some combinations of nanoparticles and liquids have been shown to substantially increase the heat transfer characteristics of the nanofluid over the base liquid.
Nanofluid heat transfer is a relatively new field being little more than a decade old. During that time, effort has been focused on determining the levels of potential thermal conductivity and heat transfer enhancements of a variety of nanofluids. In these investigations, the emphasis was usually on the magnitude of the thermal phenomena and not on the viability of the fluids for commercial applications. The thermal conductivity of nanofluids in particular has received considerable attention by researchers. Thermal conductivity is easier to measure than the heat transfer coefficient and has been used as an indicator of nanofluid heat transfer enhancement.
Enhancements in the thermal conductivities of nanofluids, for the most part, follow the predictions based on Maxwell's mean field theory assuming low concentrations and spherical nanoparticles or the effective medium theory (EMT). For small nanoparticle concentrations, EMT predicts thermal conductivity enhancement as (κf/κbf)≈1+3φ, where κf and κbf are thermal conductivities of the nanofluid and the base fluid, respectively, and φ is the nanoparticle volume fraction. However, there are instances where the actual enhancements are significantly higher than EMT predictions at very low concentrations of nanoparticles. These anomalous enhancements have typically been reported for metallic nanoparticles in fluids. Modest thermal conductivity enhancements over EMT predictions can also be achieved by modifying the shape of the nanoparticles.
Thermal conduction in nanofluids has been attributed to a variety of mechanisms, including Brownian motion, interactions between the nanoparticles and the fluid, clustering and agglomeration. There is no clear consensus on a specific mechanism; however, the general belief is that a combination of mechanisms may be operating and would be specific to a nanoparticle/fluid system and test conditions. Further, the effect of interface layers on the nanoparticles on thermal conductivity is not clearly understood. A metal particle with surface oxidation, for example, may increase the interfacial resistance and consequently reduce the thermal conductivity.
Experimental results from various nanofluid research efforts have considered a number of parameters, including without limitation: (1) particle volume concentration, (2) particle material, (3) particle size, (4) particle shape, (5) base fluid, (6) temperature, (7) additive, and (8) pH. These studies have shown heat transfer enhancement results, based on Nusselt number, to be generally in the 15-40% range for particle volume concentrations up to 4%. Some research has found that the heat transfer enhancement was close to or somewhat above predictions from standard liquid heat transfer correlations using the nanofluid properties. Nusselt number enhancement of 40% is attractive to many applications, if the nanofluid is commercially viable.
However, studies of thermal phenomena in nanofluids have generally failed to make detailed characterizations of the fluids. For instance, it is known that particle agglomeration may occur in many nanofluids so that the nominal particle size in a powder is often not the size in the suspension. In fact, particle size distributions often exist in nanofluids but are seldom measured. As a result, literature data based on nominal particle size, may in fact have involved significantly different average particle sizes and distributions in suspension.